1. Field of the Invention
The invention relates to a liquid crystal compound, a liquid crystal composition, and a liquid crystal display device. More specifically, it relates to a styrene compound, a liquid crystal composition containing the compound and having a nematic phase and a liquid crystal display device containing the composition.
2. Related Art
Based on an operation mode of liquid crystal, liquid crystal display devices are classified into phase change (PC), twisted nematic (TN), super twisted nematic (STN), bistable twisted nematic (BTN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment (VA) and the like. Based on a driving mode of an element, liquid crystal display devices can also be classified into a passive matrix (PM) and an active matrix (AM). PM is further classified into static, multiplex and the like, and AM is classified into a thin film transistor (TFT), a metal insular metal (MIM) and the like.
Ordinary properties that are necessary for a compound as a component of the composition include: (1) chemical and physical stability, (2) a high clearing point (the clearing point is a transition temperature of a liquid crystal phase and an isotropic phase), (3) a low lower limit temperature of a liquid crystal phase (the liquid crystal phase means a nematic phase, a smectic phase and the like), (4) a small viscosity, (5) a suitable optical anisotropy, (6) a suitable dielectric anisotropy (a compound having a large dielectric anisotropy often has a large viscosity) and (7) a large specific resistance.
The composition is prepared by mixing various kinds of compounds that preferably have good miscibility with each other. Since a device is used at subfreezing temperature in some cases, compounds having good compatibility with each other at low temperatures are preferably used. A compound having a high clearing point or a low lower limit temperature of a liquid crystal phase contributes to a wide temperature range of a nematic phase of the composition. A preferred composition has a small viscosity and an optical anisotropy suitable for the mode of the device. A large dielectric anisotropy of the compound contributes to a low threshold voltage of the composition. By using the composition, a device having desirable characteristics can be obtained including, for example, a wide usable temperature range, a short response time, a large contrast ratio, a small driving voltage, a small electric power consumption and a large voltage holding ratio.
As a compound having a benzene ring having an alkyl group, the following compound (A) has been reported (for example, in JP H10-291945 A), but the compound has poor liquid crystallinity.
